As a drill for drilling holes, there are for example those in which an inner insert and an outer insert are detachably attached to the tip end of a holder so that their respective rotation loci are partially overlapped with each other. Among others, those in which the inner insert and the outer insert have the same shape are frequently used. That is, the drill in which one type of drill insert (hereinafter referred to as “insert” in some cases) is detachably attached to each of the inner side and the outer side at the tip end of the holder is frequently used.
The inserts used for this drill include an inner cutting edge and an outer cutting edge. The inner cutting edge is the cutting edge for mainly cutting (machining) an inner portion of a bottom face of a hole when it is used as the inner insert. The outer cutting edge is the cutting edge for mainly cutting an outer portion of a bottom face of a hole when it is used as the outer insert.
The insert described in Japanese Unexamined Patent Application Publication No. 10-180521 has the inner cutting edge and the outer cutting edge adjacent to each other which are formed at the intersection portion between the upper face and the side face. An inner breaker groove is formed in a recess shape along the inner cutting edge, and an apex portion is formed along the inner breaker groove. An outer breaker groove is formed in a recess shape along the outer cutting edge, and an apex portion is formed along the outer breaker groove.
One of these inserts and the other are respectively attached as the inner insert and the outer insert to an inner peripheral insert pocket and an outer peripheral insert pocket formed at the tip end portions of a substantially columnar holder. The hole drilling of a work material is carried out with both cutting edges by rotating the holder around the central axis of the holder.
Chips generated during the hole drilling are treated through the inner breaker groove and the outer breaker groove formed in substantially the same shape. However, the rotational speed of the inner cutting edge is different from the rotational speed of the outer cutting edge. Therefore, the chip shape generated by the inner cutting edge and the chip shape generated by the outer cutting edge differ widely from one another.
That is, the chips generated by the inner cutting edge have a spiral shape which is a three-dimensionally complicated shape. The chips generated by the outer cutting edge have a spring-like curled shape. Particularly, when machining a work material having excellent ductility, such as stainless steels or low carbon steels, the chips generated by the outer cutting edge under high rotational speed are hardly curled, so that they are likely to extend without being cut and likely to cling to the holder during the machining. There has been the problem that these chips cannot be smoothly discharged through the inner breaker groove and the outer breaker groove.